The present invention relates to semiconductor devices, and more particularly to nonvolatile memory devices and methods of fabricating the same.
In general, semiconductor memory devices can be classified into volatile and nonvolatile types. Nonvolatile memory devices, including dynamic random access memories (DRAMs) and static random access memories (SRAMs), can be conducive in fast data input/output operation, but will lose their data when there is no power supply. On the contrary, the nonvolatile memory devices can retain their data even without power supply.
Flash memory devices, which are a generally classified as nonvolatile memory devices, are highly integrated devices that can have the merits of erasable and programmable read-only memories (EPROMs) and electrically EPROMs.
The flash memory devices are generally distinguished into NOR and NAND types. A NOR-type flash memory can be operable at high frequencies since the memory cells can be controlled independently. A NOR-type flash memory device, however, is larger than that of NAND-type memory device because a bitline contact is required for each pair of memory cells.
Many techniques have been proposed to address the shortcomings of the NOR-type flash memory. For example, self-aligned source line (SAS) and shallow trench isolation (STI) techniques can be useful to reduce memory cell sizes in the bitline and wordline directions, respectively. These techniques can be currently used in the fabrication of NOR-type flash memory devices.
However, with a reduction of the size of memory cells, the SAS resistance can increase substantially. In addition, as trenches are deepened using a high internal voltage in the flash memory device, the SAS resistance can increase further. While an increasing concentration of impurities can be injected into the SAS to overcome this effect, the memory device may experience reduced reliability due to a channel length shortening, which can cause a punch-through effect.